In a small bowl, whisk together the flour, salt, and instant yeast. Add the water.
Using a rubber spatula, mix until the liquid is absorbed, and the ingredients form a sticky dough ball.
Rub the surface of the dough lightly with olive oil.
Cover the bowl with a cloth or plastic wrap and place in the refrigerator for 15 minutes.
Using your damp hands or two forks, deflate the dough by releasing it from the sides of the bowl and pulling it toward the center. Rotate the bowl in quarter turns as you work the dough. Cover and set aside for at least one hour.
Lightly oil 4 bun rings on a baking sheet. Divide the focaccia into 4 equal sizes. Pour the focaccia dough into each ring. Do not fill the ring!
Cover and allow dough to rise at room temperature for at least 60 minutes.
Preheat oven to 400°F. Remove plastic wrap. Gently poke dough all over with wet fingertips, pressing just enough to leave indentations in top of dough. Top the dough with olive oil, sesame seeds, thinly sliced onions, and salt.
Bake 15-20 minutes or until top is golden brown. Cool in rings 5 minutes. Remove rings, slice, and build your burger. Enjoy!